The most important orbitals in molecules are the frontier molecular
orbitals, called highest occupied molecular orbital (HOMO)
and lowest unoccupied molecular orbital (LUMO). These orbitals
determine the way the molecule interacts with other species. The
frontier orbital gap helps to characterize the chemical reactivity
and kinetic stability of the molecule. A molecule with a small
frontier orbital gap is more polarizable and is generally associated
with a high chemical reactivity, low kinetic stability and is also
termed as soft molecule [13e15]. The low values of frontier orbital
gap in Lornoxicam make it more reactive and less stable. The HOMO
is the orbital that primarily acts as an electron donor and the LUMO
is the orbital that largely acts as the electron acceptor. The energy
gap between HOMO and LUMO is a critical parameter in determining
molecular electrical transport properties [16]. The 3D plots
of the frontier orbitals HOMO and LUMO for Lornoxicam are shown
in Fig. 2. The positive phase is red and the negative one is green. It
can be seen from the figure that, the HOMO levels are spread over
the entire molecule except the CH groups C18, H32 and C16, H30.
The LUMO in Lornoxicam is found to spread over the entire molecule
except CH3 group